Document diagostic attribute duplicates

[mejrs]

You can stack these attributes, which I think is a cool little known fact. For example:

#[diagnostic::on_unimplemented(
    message = "pyclass `{Self}` cannot be subclassed",
    label = "required for `#[pyclass(extends={Self})]`",
    note = "`{Self}` must have `#[pyclass(subclass)]` to be eligible for subclassing"
)]
#[cfg_attr(
    Py_LIMITED_API,
    diagnostic::on_unimplemented(
        note = "with the `abi3` feature enabled, PyO3 does not support subclassing native types",
    )
)]
pub trait PyClassBaseType: Sized {

See rust-lang/rust#155294 for the test this rule will link to.

[mejrs]

View changes since the review

Existing prose looks like

The `cfg` attribute may be used any number of times on a form.

and

The `derive` attribute may be used any number of times on an item.

I'm not sure what I should use, or what a form is for that matter (it's not in the glossary).

[ehuss]

Thanks for the PR! We have an open PR at #1893 that updates this rule (among a few other things).

[mejrs]

Oh shoot, I didn't see that at all. Great minds think alike, I suppose. Would you like me to merge the linked test?